This invention relates to a disaster prevention monitoring and control facility.
A disaster prevention monitoring and control facility is known in the art in which a receiver polls terminal units such as fire sensors, fire detectors, smoke eliminators, fire extinguishers, antitheft detectors or repeaters connected with those units to allow the called terminal unit to transmit monitoring information to the receiver, and the receiver to collect the monitoring information. This facility transmits control information to the terminal unit called by the receiver.
If a transmission error occurs between a receiver and a terminal unit in the disaster prevention monitoring and control facility of this type, there is a possibility, for example, that an area where there is no fire is erroneously judged, as an area where a fire has broken out or a terminal unit may erroneously evaluate a control command to operate a smoke eliminator or a fire extinguisher.
Thus, the prior-art disaster prevention monitoring and control facility described above performs a parity check to prevent transmission errors in a signal from the receiver to the terminal unit or from the terminal unit to the receiver from transmission errors.
However, even though a parity check can detect a one bit error, it cannot detect a plurality of bit errors.
For preventing the erroneous bit evaluation the continuous transmission of the same signals twice from the receiver and the terminal unit and the receiving terminal unit or the receive to determine whether or not the two received signals coincide or not has been considered. In this case, there is a drawback in that the frame length of the transmitted signal is increased between the receiver and the terminal unit to increase the time needed to poll all the terminal units, thereby taking a longer time to detect a fire or intrusion which demand short processing times.
Further, the terminal units of prior-art polling type facilities only return data to the receiver. Thus, there is another drawback in that the receiver cannot accurately know the type of data transmitted from the terminal unit.
If the terminal unit, for instance, erroneously returns the data of a tested result when the receiver, for example, instructs the terminal unit to return the data detected by a certain sensor, the receiver identifies the returned data as the data detected by the sensor. Thus, in this case, the receiver erroneously judges the data.
Moreover, if a polled terminal unit does not reply in the prior-art disaster prevention monitoring and control facility, the terminal unit is continuously called a predetermined number of times (e.g., five times), and the next terminal unit is thereafter polled. When the final terminal unit is thus polled, a first polling cycle is finished. If the terminal unit which did not reply in the first cycle does not again respond in the second cycle, it is repeatedly polled a predetermined number of times (five times in this example) until a response is obtained.
If a terminal unit which has transmitted an error signal is polled, a process is executed which is similar to the case of no response from the terminal unit. In other words, when the terminal unit which has transmitted an error signal is polled, the terminal unit is continuously called a predetermined number of times, the next terminal unit is thereafter polled, and if the terminal unit which has returned an error signal in the first polling cycle also returns an error signal in the second cycle, then the polling steps are repeated a predetermined number of times until a normal signal is returned.
However, in the abovementioned disaster prevention monitoring and control facility, there arises a drawback in that the polling steps stagnate in the terminal unit which has not responded or has transmitted an erroneous signal to delay the acquisition of monitoring information or the transmitting speed of control information. When there are a plurality of abnormal terminal units which have not replied or have transmitted erroneous signals and the acquisition of monitoring information or the transmission of control information is required for an emergency such as a fire or a theft, a delay in the acquisition speed or the transmission speed becomes a serious problem.